Scientists say they shrank when dinosaurs took to skies as birds and competed with them

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This fossilized insect wing from the species Stephanotypus schneideri is about 300 million years old. The wing is about 7.5 inches (19 centimeters long), substantially smaller than the largest fossil insect (Meganeuropsis permiana, about 33 cm long). Superimposed on the fossil is a drawing of the largest Cenozoic insect (it's about 12 million years old), Epiaeschna lucida, which comes in at 2.6 inches (6.7 cm) long, similar to modern insects.

Millions of years ago, oversized insects such as griffinflies boasting wingspans comparable to today's hawks scuttled across (and fluttered above) the planet. But why these jumbo jets of the insect world shrank to modern size has remained a mystery, until now.

Turns out, as dinosaurs evolved flight and eventually took to the skies as birds, they beat down the huge insects already living there, effectively putting a cap on insect size through predation and competition in the prehistoric skies, as birds developed into sophisticated flying machines.

"The change in insect size is gradual," study researcher Matthew Clapham of the University of California, at Santa Cruz, told LiveScience. "This gradual change fits quite nicely with the gradual evolution in birds at the time."

Insects during the Permian era (about 290 million to 250 million years ago) were huge compared with their counterparts today, boasting wingspans up to 30 inches (70 centimeters) across. The high levels of oxygen in the prehistoric atmosphere helped fuel their growth.

To figure out why modern insects are no longer so big, the researchers compiled a database of wingspan measurements of 10,500 fossilized insects from the last 320 million years of insect evolution. The authors found that during the first 150 million years of insect evolution, the wingspans on these insects mirrored levels of atmospheric oxygen; the more oxygen in the atmosphere, the larger the insects the environment could support.

These flying insects need lots of oxygen to support their flight muscles, Clapham said, and since their breathing tubes are inefficient, they need high atmospheric oxygen levels to grow large. The more oxygen in the environment, the more muscle mass the insect can provide oxygen for and the larger the insect can be.

Around 140 million years ago, though, things changed. Insect wingspan stopped depending on oxygen levels and started dropping.

The researchers noticed that this change happened around the same time that birds first took to the skies. About 150 million years ago, the "first bird" Archaeopetryx appeared, and about 25 million years later, the lineage had diversified greatly.

Over time, the birds developed physical features that enabled quick flying and better maneuvering. As a result, they started eating the large insects, and competing with them for food sources.

"These large insects are predatory, and they eat smaller insects. It's possible that birds may have also eaten small insects, as many modern birds do," Clapham said. "If they are competing for the same resource, birds may have been better competitors for these food sources."

A second slump in insect size happened about 60 million years ago, which the researchers think could be related to the evolution of bats, further increases in birds' aerial abilities, or to the mass extinction event that killed the dinosaurs.

The study is detailed Monday in the journal Proceedings of the National Academy of Sciences.

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